Standards exist which establish seat leakage classifications for control valves according to a valve's ability to shut off flow when it is closed. These standards specify the amount of flow allowed through the valve at shut off. This amount of flow is usually measured as a percentage of the rated valve capacity when a specified differential pressure is applied across the inlet and outlet ports of the valve. For example, ANSI standards provide that, for Leakage Class II, the maximum seat leakage through a valve shall not exceed 0.5% of its rated valve capacity at shut off. Similarly, Leakage Classes III and IV permit seat leakages of no more than 0.1% and 0.01%, respectively, of rated valve capacity to flow through the valve at shut off.
In order to determine whether or not a particular valve design meets the requirements of Leakage Classes II, III and IV, an air or water leakage test is performed on the valve while the valve is in its closed position. Typically, this air or water leakage test is performed by coupling an air or water supply to the inlet port of a closed valve and measuring the amount of leakage through the valve.
On the other hand, a valve which satisfies the requirements of Leakage Class V is tested using a water leakage test wherein a source of water is coupled to the inlet port of the valve and the amount of water flowing through the valve while it is in its closed position is measured. According to Leakage Class V, the maximum seat leakage through the valve is only 0.0005 milliliters of water per minute, per inch of port diameter, per psi differential pressure from inlet port to outlet port. As an example, if the valve has two inch diameter ports and the source of water connected to the inlet port is at 100 psi with respect to the pressure at the outlet port, then 0.1 milliliter of water is permitted to flow through the valve per minute when the valve is in its closed position. If the flow exceeds this amount, the valve does not meet the requirements of ANSI Leakage Class V.
In addition to meeting the requirements of the desired ANSI Leakage Class, it is also advantageous for the valve to have a balanced valve plug. A balanced valve plug is one having substantially equal areas against which the internal fluid pressures of the valve act so that the net forces acting on the valve plug as a result of these pressures will be minimized. By minimizing these net forces, the size of the actuator necessary to overcome these net forces and move the valve plug is also minimized.
A prior art double port valve such as shown in FIG. 1 is often used to balance the net forces acting on the valve plug to thereby minimize the actuator force necessary to position the plug. The double port valve shown in FIG. 1 has a valve plug 11 carrying two lands 12 and 13. The land 12 cooperates with a seat ring 14 at the upper port and the land 13 cooperates with a seat ring 15 at the lower port to control the flow of fluid from an inlet 16 to an outlet 17.
The surface areas of the lands 12 and 13 cannot be made exactly equal because, when the plug 11 is inserted through the top of the valve during valve assembly, the land 13 must be small enough that it can pass through the seat ring 14. On the other hand, the land 12 must be larger than land 13 so that land 12 does not pass through the seat ring 14 but instead seats against seat ring 14. Thus, the land 12 is larger than the land 13.
Because the area of the land 12 is larger than the land 13, the fluid pressure from the inlet 16 near or at shut off will exert a greater force on the land 12 than on the land 13. The resulting net force is in a direction to oppose closing of the valve. This net force must be overcome by the actuator in order to close the valve.
The surface areas of the lands 12 and 13 are, however, nearly equal so that the net force exerted on the valve plug 11 is small. However, because it is very difficult to machine the lands and seat rings to the tight tolerances necessary to prevent leakage at shut off and to maintain thermal expansion differences at a minimum, it is consequently very difficult to manufacture this double port valve so that it will meet the maximum leakage requirements of Leakage Class V, especially at high operating temperatures.
Shown in FIG. 2 is a prior art double seat valve which is designed to meet the leakage requirements of Leakage Class V and to minimize the size of the valve's actuator. The valve shown in FIG. 2 has a valve plug 21 positioned by a valve stem 22 which enters the valve through a typical bonnet 23 and a valve packing 24. The valve plug 21 has a two part construction which includes a pilot plug 25 and a primary plug 26. The primary plug 26 cooperates both with openings 27 in a cage 28 and with a lower valve seat in the form of a seat ring 31 to control the flow of fluid between valve ports 29 and 30. The primary plug 26 is moved away from the seat ring 31 by virtue of a washer 32 attached to the end of the pilot plug 25 by a nut 33 and is moved toward the seat ring 31 by virtue of a plurality of springs only two of which, springs 37 and 38, are shown.
The pilot plug 25 has openings 35 and 36 therein so that the fluid pressures above and below the pilot plug 25 will be equal. Thus, when the valve is to be opened, the forces acting on the pilot plug 25 caused by fluid pressure on both of its sides are relatively balanced. The valve stem 22 lifts the pilot plug 25 away from an upper valve seat 34 formed in the primary plug 26 to allow the pressure on both sides of the primary plug 26 to equalize through openings 39 in the primary plug 26.
When the valve is to be closed, the valve stem 22 drives the primary plug 26 in a direction so that it will seat against the seat ring 31. The springs 37 and 38 exert a force between the pilot plug 25 and the primary valve plug 26 to keep the port between the pilot plug 25 and the upper valve seat 34 open to maintain the pressures across the primary plug 26 balanced until the primary plug 26 is seated against the lower valve seat 31. When the primary plug 26 is seated against the lower valve seat 31, the valve stem 22 will cause the pilot plug 25 to seat against the upper valve seat 34 completing closure of the valve.
Thus, the force necessary to move the primary plug 26 to, and away from, the seat ring 31 is minimized, and minimum actuator size is needed to open and/or close the valve of FIG. 2. This double seat arrangement allows the valve to achieve Leakage Class V shut off. However, the valve construction shown in FIG. 2 is complicated, requires a plurality of springs, and contains numerous parts which require tight machining tolerances.